Calibrating elevator actuator for disk drive

ABSTRACT

A data storage device is disclosed comprising a first disk comprising a first disk surface, a second disk comprising a second disk surface, an elevator actuator configured to actuate a head along an axial dimension relative to the first and second disks, a radial actuator configured to actuate the head radially over the first disk surface or the second disk surface, and a position sensor configured to generate a sinusoidal sensor signal representing a position of the head along the axial dimension. A crashstop_offset along the axial dimension is measured from a crashstop position of the elevator actuator to a zero crossing of the sinusoidal sensor signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/851,169, filed on May 22, 2019, which is hereby incorporated by reference in its entirety.

BACKGROUND

Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.

A disk drive typically comprises a plurality of disks each having a top and bottom surface accessed by a respective head. That is, the VCM typically rotates a number of actuator arms about a pivot in order to simultaneously position a number of heads over respective disk surfaces based on servo data recorded on each disk surface. FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6 ₀-6 _(N) recorded around the circumference of each servo track. Each servo sector 6 _(i) comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6 _(i) further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.

FIGS. 2A-2C show a data storage device in the form of a disk drive according to an embodiment comprising an elevator actuator configured to actuate a head along an axial dimension relative to first and second disks, a radial actuator configured to actuate the head radially over the disk surfaces, and a position sensor configured to generate a sinusoidal sensor signal representing a position of the head along the axial dimension.

FIG. 3A shows an embodiment wherein a crashstop_offset is measured along the axial dimension from a crashstop position of the elevator actuator to a zero crossing of the sinusoidal sensor signal.

FIG. 3B shows a sinusoidal sensor signal generated by the position sensor according to an embodiment.

FIG. 4 is a flow diagram according to an embodiment wherein the crashstop_offset is used to position the head relative to the first disk surface.

FIG. 5A shows an embodiment wherein a first elevator actuator comprising a first lead screw having a first pitch actuates an actuator arm along the axial dimension, and a second elevator actuator comprising a second lead screw having a second pitch actuates at least part of a load/unload ramp along the axial dimension.

FIG. 5B is a flow diagram according to an embodiment wherein the first pitch of the first elevator actuator and the second pitch of the second elevator actuator are measured in order to synchronize the simultaneous movement of both actuators.

FIGS. 6A-6D show a disk drive according to an embodiment comprising a first elevator actuator configured to actuate two actuator arms along an axial dimension relative to multiple disks, and a second elevator actuator configured to actuate at least part of a ramp along the axial dimension.

FIGS. 7A and 7B show an embodiment wherein the position sensor comprises two sensor elements configured to generate quadrature sinusoids in order to compensate for amplitude variations in the sensor signals.

DETAILED DESCRIPTION

FIGS. 2A-2C show a data storage device in the form of a disk drive according to an embodiment comprising a first disk comprising a first disk surface 16A and a second disk comprising a second disk surface 16C. An elevator actuator 20 is configured to actuate a head 18A along an axial dimension relative to the first and second disks, and a radial actuator 22 configured to actuate the head 18A radially over the first disk surface 16A or the second disk surface 16C. A position sensor 24 is configured to generate a sinusoidal sensor signal representing a position of the head 18A along the axial dimension. The disk drive further comprises control circuitry 26 configured to measure a crashstop offset (represented as crashstop_offset in the description below) along the axial dimension from a crashstop position of the elevator actuator 20 to a zero crossing of the sinusoidal sensor signal.

In the embodiment of FIG. 2A, each disk surface comprises a plurality of servo sectors 28 ₁-28 _(N) that define a plurality of servo tracks, wherein data tracks 30 are defined relative to the servo tracks at the same or different radial density. The control circuitry 26 processes a read signal 32 emanating from the head 18A to demodulate the servo sectors and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. A servo control system in the control circuitry 26 filters the PES using a suitable compensation filter to generate a control signal 34 applied to a VCM 22 which rotates an actuator arm 36A about a pivot in order to actuate the head radially over the disk surface in a direction that reduces the PES. In one embodiment, the head 18A may be actuated over the disk surface 16A based on the PES using one or more secondary actuators, for example, a microactuator that actuates a suspension coupling a head slider to the actuator arm 36A, or a microactuator that actuates the head slider relative to the suspension (e.g., using a thermal actuator, piezoelectric actuator, etc.). The servo sectors 28 ₁-28 _(N) may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern (FIG. 1).

In the embodiment shown in FIGS. 2B-2C, the VCM 22 actuates two actuator arms 36A and 36B each actuating a respective head 18A and 18B. The elevator actuator 20 in this embodiment actuates the actuator arms 36A and 36B in an axial dimension relative to the disks, effectively implementing an “elevator” system that raises/lowers the actuator arms 36A and 36B to enable the heads 18A and 18B to access the top and bottom surfaces of multiple disks (two disks in this example). This embodiment reduces the cost of the disk drive by reducing the number of actuator arms as well as the number of heads needed to access disk surface as compared to a conventional disk drive employing multiple actuator arms for actuating a respective head over a respective (dedicated) disk surface. This embodiment may also reduce the cost of the VCM 22 since it reduces the number (mass) of actuator arms rotated about the pivot. Another embodiment may employ a single actuator arm for actuating two heads or a single head, thereby further reducing the cost of the disk drive. In yet another embodiment, a different type of radial actuator may be employed to actuate the head(s) radially over the disk surfaces, such as a linear actuator.

Any suitable position sensor 24 may be employed in the embodiments disclosed herein, such as any suitable optical or magnetic sensor. In addition, the position sensor may be implemented in any suitable mechanical configuration, such as an encoder strip and a corresponding transducer coupled to the actuator arm assembly as shown in the embodiment of FIG. 2B. FIG. 3A shows an embodiment wherein the position sensor 24 comprises a magnetic encoder strip 38 comprising a plurality of alternating polarity fixed magnets (N/S, N/S, . . . ). The magnetic encoder strip 38 is fixed relative to the actuator arms and a suitable magnetic sensor, such as a Hall effect sensor, is coupled, for example, to the actuator arms 36A and 36B. As the elevator actuator 20 moves the actuator arms 36A and 36B in the axial dimension in order to reposition the head(s) over different disk surfaces, the magnetic sensor generates a sinusoidal sensor signal such as shown in FIG. 3B due to sensing the alternating magnetic field of the magnetic encoder strip 38.

In one embodiment, the encoder strip 38 such as shown in FIG. 3A as well as the disks may be installed during manufacturing such that the axial location of each disk surface relative to the encoder strip 38 may be known within an insignificantly small variance. However in one embodiment, the variance in coupling the actuator arms to the base of the disk drive may create a significant variance between the location of the actuator arms relative to the encoder strip 38 when the elevator actuator 20 reaches a crashstop position. In the example of FIG. 3A, the crashstop position may be defined as the elevator actuator 20 moving the actuator arms down the axial dimension to the lowest point. In one embodiment, the elevator actuator 20 may contact a physical crashstop at this point, and in another embodiment, at least part of the actuator arm assembly may contact a physical crashstop. In one embodiment, the variance between the location of the actuator arms relative to the encoder strip 38 at the crashstop position is calibrated out by detecting a zero crossing in the sinusoidal sensor signal which then defines a home position for the actuator arms relative to the encoder strip 38.

An example of this embodiment is shown in FIG. 3A wherein a crashstop_offset is measured along the axial dimension from a crashstop position of the elevator actuator 20 to a zero crossing of the sinusoidal sensor signal. In the example of FIGS. 3A and 3B, the elevator actuator 20 moves the actuator arms up the axial dimension until the first zero crossing in the sinusoidal sensor signal is detected. In other embodiments, the crashstop_offset may be measured relative to a different (e.g., second) zero crossing in the sinusoidal sensor signal. In one embodiment, the crashstop_offset may be measured by detecting a target zero crossing in the sinusoidal sensor signal, and then moving the elevator actuator 20 to the crashstop position. In one embodiment, the crashstop_offset may be measured by moving the elevator actuator 20 between the crashstop position and the target zero crossing multiple times and averaging the offset measurements. In yet another embodiment, the crashstop_offset may be measured by detecting multiple zero crossings in the sinusoidal sensor signal. For example, in the example of FIG. 3A the elevator actuator 20 may move the actuator arms up the axial dimension over multiple zero crossings and the crashstop_offset (e.g., relative to the first zero crossing) may be measured based on the measured position of the multiple zero crossings (e.g., by averaging out noise in the sinusoidal sensor signal).

In one embodiment, after measuring the crashstop_offset relative to a target zero crossing, the elevator actuator 20 is controlled to move the head(s) to a nominal position representing the location of a target disk surface (e.g., move head 18B to disk surface 16D in FIG. 2C). In one embodiment, the nominal position is defined as a nominal distance D1 from the crashstop position to the position representing the target disk surface. Accordingly in one embodiment, the elevator actuator 20 is first moved to the target zero crossing described above which represents the home position for the position sensor 24, and then the elevator actuator 20 is moved a distance: D1−crashstop_offset in order position the head(s) to the nominal position representing the location of the target disk surface. An example of this embodiment is understood with reference to the flow diagram of FIG. 4, wherein the elevator actuator 20 is moved to the crashstop position (block 40), and then the elevator actuator 20 moves the heads along the axial dimension (block 42) until the first zero crossing is detected in the sinusoidal sensor signal (block 44). The corresponding crashstop_offset is saved (block 46), and the position sensor is “homed” based on the detected zero crossing (block 48). For example, the “home” position may be defined as a distance of zero along the axial dimension which may be defined by the location of the detected zero crossing in the sinusoidal sensor signal. The elevator actuator 20 then moves the head(s) from the home position to the nominal position of a target disk surface by moving the head(s) by a distance of D1 minus the crashstop_offset (block 50).

In one embodiment, the measured crashstop_offset may be saved, for example, in a non-volatile semiconductor memory and then used to move the head(s) to a first target disk surface when the disk drive is powered on. For example, when the disk drive is powered on the elevator actuator 20 may be moved to the crashstop position and then moved up until detecting the first zero crossing in the sinusoidal sensor signal which defines the home position for the elevator actuator. The elevator actuator 20 then moves the heads by the distance D1 minus the saved crashstop_offset (i.e., in this embodiment it is unnecessary to remeasure the crashstop_offset).

In one embodiment, the elevator actuator 20 may be controlled open loop when moving to the crashstop position as well as moving to the target zero crossing of the sinusoidal sensor signal as described above with reference to FIGS. 3A and 3B. After homing the position sensor 24 based on the detected zero crossing, in one embodiment the elevator actuator 20 may be controlled closed loop based on any suitable states (velocity, position, etc.) as determined from the position sensor 24. For example, in one embodiment the elevator actuator 20 may be controlled using a suitable velocity profile in order to seek the head(s) along the axial dimension toward a target position relative to the disk surfaces. When the head(s) are within a predetermined threshold of the target position, the closed loop control may switch the feedback in order to control the elevator actuator 20 based on a position error in order to settle onto the target position.

In one embodiment (an example of which is shown in FIGS. 6A-6D), the disk drive may comprise a ramp 52 configured to load/unload the head(s) to and from a target disk surface. In one embodiment, the actuator arm(s) may be actuated along the axial direction by a first elevator actuator 54, and at least part of the ramp 52 may be simultaneously actuated along the axial dimension by a second elevator actuator 56 in order to position the head(s) relative to the disk surfaces prior to loading the head(s) onto a target disk surface. In the embodiment shown in FIGS. 6A-6D, the first elevator actuator 54 comprises a first stepper motor configured to rotate a first lead screw 58 in order to actuate the actuator arms 36A and 36B which are threaded onto the first lead screw 58, and the second elevator actuator 56 comprises a second stepper motor configured to rotate a second lead screw 60 in order to actuate at least part of the ramp 52 which is threaded onto the second lead screw 60. In one embodiment shown in FIG. 5A, the first lead screw 58 comprises a first pitch and the second lead screw 60 comprises a second pitch different than the first pitch. In the example of FIG. 5A, the pitch of the first lead screw 58 is greater than the pitch of the second lead screw 60, but in other embodiments the opposite may be the case.

In one embodiment, the control circuitry 26 synchronizes a simultaneous movement of the first and second elevator actuators 54 and 56, for example, to compensate for the difference in pitch between lead screws such as shown in FIG. 5A. For example, in one embodiment a first velocity command may be generated to move the first elevator actuator 54 and a corresponding second velocity command may be generated to simultaneously move the second elevator actuator 56. In one embodiment, the second velocity command may initially be generated based on a default (nominal) pitch ratio between the first and second lead screws 58 and 60: VELcmd1=VEL1 VELcmd2=VELcmd1*P1def/P2def where P1def represents the default pitch of the first lead screw 58 and P2def represents the default pitch of the second lead screw 60. After homing the first and second elevator actuators 54 and 56 as described above, the actual pitch of each lead screw 58 and 60 is measured, and the measured pitches are used to generate the velocity commands for simultaneously and synchronously moving the first and second elevator actuators 54 and 56: VELcmd1=VEL1 VELcmd2=VELcmd1*P1mes/P2mes where P1mes represents the measured pitch of the first lead screw 58 and P2mes represents the measured pitch of the second lead screw 60.

An example of this embodiment is understood with reference to the flow diagram of FIG. 5B, wherein default velocity commands are generated for the first and second elevator actuators (block 62) based on the ratio of the default pitches of the lead screws 58 and 60 as described above. Each elevator actuator is moved to their respective crashstop positions using their respective default velocity commands (block 64). Each elevator actuator is then moved up toward a first zero crossing of at least one of the sinusoidal position signals (block 66) until the first zero crossing is detected (block 68). The position sensors are homed (e.g., zeroed) based on the detected zero crossing (block 70). Both axial motors are controlled to rotate n full revolutions (block 72), and the corresponding displacements Dn1 and Dn2 of the actuator arm(s) and ramp are measured based on the corresponding position sensors (block 74). The pitches of each lead screw is then measured (block 76). based on: P1mes=Dn1/n P2mes=Dn2/n where P1mes represents the measured pitch of the first lead screw and P2mes represents the measured pitch of the second lead screw. The velocity command for the second elevator actuator is then generated based on the ratio of the measured pitches (block 78). In an alternative embodiment, a velocity command may be generated for the second elevator actuator and a corresponding velocity command generated for the first elevator actuator based on the ratio of the measured pitches.

FIGS. 6A-6D show a disk drive according to an embodiment comprising a first elevator actuator 54 (FIG. 6B) configured to actuate two actuator arms 36A and 36B along an axial dimension relative to multiple disks, and a second elevator actuator 56 (FIG. 6D) configured to actuate at least part of a ramp along the axial dimension. The first elevator actuator 54 comprises a first stepper motor configured to rotate a first lead screw 58 (FIG. 6C) in order to vertically actuate the actuator arms 36A and 36B which are threaded onto the first lead screw 58, and the second elevator actuator 56 comprises a second stepper motor configured to rotate a second lead screw 60 (FIG. 6D) in order to vertically actuate at least part of the ramp 52B which is threaded onto the second lead screw 60. In the embodiment of FIG. 6D, the ramp 52 comprises a first ramp part 52A that is fixed relative to the disks, and a second ramp part 52B that is vertically actuated in the axial dimension by rotating the second lead screw 60. The heads 18A and 18B are unloaded onto the second ramp part 52B prior to vertically actuating the second ramp part 52B in the axial dimension. After positioning the second ramp part 52B to a target disk, the heads 18A and 18B are loaded onto the respective disk surfaces by rotating the actuator arms 36A and 36B such that the heads slide along the second ramp part 52B onto the first ramp part 52A, and then loaded from the first ramp part 52A onto the respective disk surfaces.

In the embodiment of FIG. 6C, the first lead screw 58 comprises a cylindrical assembly that is rotated (clockwise or counter-clockwise) about a pivot assembly 80 using any suitable stepper motor (e.g., a claw-pole permanent magnet stepper motor), thereby adjusting the vertical position of the actuator arms 36A and 36B along the axial dimension. A voice coil 82 is coupled to the pivot assembly 80 which is rotated about a fixed pivot 84 in order to rotate the combined assembly (voice coil 82, actuator arms 36A and 36B, and lead screw 58) about the fixed pivot 84, thereby actuating the heads 18A and 18B radially over the respective disk surfaces. The combined assembly further comprises an encoder strip 86 (similar to the encoder strip 38 of FIG. 3A) coupled to the voice coil assembly and at least one sensor 88 coupled to the actuator arm assembly. As the actuator arms 36A and 36B move along the axial dimension, the sensor 88 generates a sinusoidal sensor signal such as shown in FIG. 3B which may be demodulated in any suitable manner to generate a position signal representing a position of the actuator arms along the axial dimension. In the embodiment of FIG. 6D, the ramp assembly also comprises an encoder strip 90 (similar to the encoder strip 38 of FIG. 3A) and at least one sensor coupled to the second ramp part 52B for generating a sinusoidal sensor signal such as shown in FIG. 3B as the second ramp part 52B moves along the axial dimension by rotating the second lead screw 62 using any suitable stepper motor.

FIGS. 7A and 7B show an embodiment wherein the position sensor comprises two sensor elements 92A and 92B (e.g., two Hall effect sensors) configured to generate respective quadrature sinusoids 94A and 94B (phase offset by ninety degrees) in order to compensate for amplitude variations in the sensor signals 92A and 92B. In one embodiment, the first quadrature sinusoid 94A generated by the first sensor element 92A may be represented as: g·sin(t) and the second quadrature sinusoid 94B generated by the second sensor element 92B may be represented as: g·cos(t) where g represents a gain of the sinusoids. In one embodiment, the gain g (and corresponding amplitude of the sinusoids) may vary due, for example, to temperature fluctuations or fluctuations in the gap between the encoder strip 86/90 and the sensor elements 92A and 92B across the stroke of the sensor elements. Variations in the amplitude of a single sinusoidal signal generated by a single sensor element may induce errors in the position signal demodulated from the sinusoidal signal. This amplitude variation can be compensated by employing two sensor elements 92A and 92B that are offset vertically by ninety degrees (such as shown in FIG. 7A) and by exploiting the trigonometry identity: sqrt[(g·sin(t))²+(g·cos(t))²]=g

That is, the amplitude of the quadrature sinusoids 94A and 94B may be normalized by dividing the output of each sensor element 92A and 92B by the above trigonometry identity:

$\frac{g \cdot {\sin(t)}}{s\; q\; r\;{t\left\lbrack {\left( {g \cdot {\sin(t)}} \right)^{2} + \left( {g \cdot {\cos(t)}} \right)^{2}} \right\rbrack}} = {\sin(t)}$ $\frac{g \cdot {\cos(t)}}{s\; q\; r\;{t\left\lbrack {\left( {g \cdot {\sin(t)}} \right)^{2} + \left( {g \cdot {\cos(t)}} \right)^{2}} \right\rbrack}} = {\cos(t)}$

In other embodiments, other anomalies in the quadrature sinusoids 94A and 94B may be compensated using any suitable signal processing techniques prior to normalizing the amplitude as described above, such as compensating for a sensitivity difference between the sensor elements 92A and 92B resulting in different relative amplitudes of the quadrature sinusoids, compensating for a phase error between the quadrature sinusoids, or compensating for a DC offset of the quadrature sinusoids.

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a data storage controller, or certain operations described above may be performed by a read channel and others by a data storage controller. In one embodiment, the read channel and data storage controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or data storage controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry. In some embodiments, at least some of the flow diagram blocks may be implemented using analog circuitry (e.g., analog comparators, timers, etc.), and in other embodiments at least some of the blocks may be implemented using digital circuitry or a combination of analog/digital circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, a hybrid disk drive, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein. 

What is claimed is:
 1. A data storage device comprising: a first disk comprising a first disk surface; a second disk comprising a second disk surface; a head; an elevator actuator configured to actuate the head along an axial dimension relative to the first and second disks; a radial actuator configured to actuate the head radially over the first disk surface or the second disk surface; a position sensor configured to generate a sinusoidal sensor signal representing a position of the head along the axial dimension; and control circuitry configured to measure a crashstop offset along the axial dimension from a crashstop position of the elevator actuator to a zero crossing of the sinusoidal sensor signal.
 2. The data storage device as recited in claim 1, wherein the zero crossing represents a home position of the head along the axial dimension.
 3. The data storage device as recited in claim 2, wherein the control circuitry is further configured to: control the elevator actuator to move the head to the home position; and after moving the head to the home position, control the elevator actuator to move the head along the axial dimension a distance: D1−crashstop_offset where D1 represents a distance along the axial dimension from the crashstop position to the first disk surface and crashstop_offset represents the crashstop offset.
 4. The data storage device as recited in claim 3, wherein the control circuitry is further configured to move the head to the home position when the data storage device is powered on.
 5. The data storage device as recited in claim 1, wherein the control circuitry is further configured to measure the crashstop offset by: controlling the elevator actuator to move the head in a first direction along the axial dimension until the crashstop position is detected; and after detecting the crashstop position, controlling the elevator actuator to move the head in a second direction opposite the first direction along the axial dimension until the zero crossing is detected.
 6. The data storage device as recited in claim 1, wherein the control circuitry is further configured to measure the crashstop offset by: controlling the elevator actuator to move the head between the crashstop position and the zero crossing multiple times to generate a plurality of measured crashstop offsets; and estimate the crashstop_offset based on the plurality of measured crashstop offsets.
 7. The data storage device as recited in claim 1, further comprising an actuator arm, wherein: the head is coupled to a distal end of the actuator arm; and the elevator actuator actuates the actuator arm along the axial dimension.
 8. The data storage device as recited in claim 1, further comprising an actuator arm and a ramp for loading/unloading the head, wherein: the head is coupled to a distal end of the actuator arm; and the elevator actuator actuates the ramp along the axial dimension.
 9. A data storage device comprising: a first disk comprising a first disk surface; a second disk comprising a second disk surface; an actuator arm; a head coupled to a distal end of the actuator arm; a ramp for loading/unloading the head; a first elevator actuator configured to actuate the actuator arm along an axial dimension relative to the first and second disks; a first position sensor configured to generate a first sensor signal representing a displacement of the actuator arm along the axial dimension; a second elevator actuator configured to actuate at least part of the ramp along the axial dimension; a second position sensor configured to generate a second sensor signal representing a displacement of the head along the axial dimension; and a radial actuator configured to actuate the head radially over the first disk surface or the second disk surface.
 10. The data storage device as recited in claim 9, further comprising control circuitry configured to move the first elevator actuator to a first crashstop position along the axial dimension based on a first default velocity command and simultaneously move the second elevator actuator to a second crashstop position along the axial dimension based on a second default velocity command different from the first default velocity command.
 11. The data storage device as recited in claim 10, wherein: the first elevator actuator comprises a first lead screw having a first pitch; the first default velocity command is based on a first nominal pitch representing the first pitch; the second elevator actuator comprises a second lead screw having a second pitch; and the second default velocity command is based on a second nominal pitch representing the second pitch.
 12. The data storage device as recited in claim 11, wherein after moving the first elevator actuator to the first crashstop position and moving the second elevator actuator to the second crashstop position, the control circuitry is further configured to: measure the first pitch of the first lead screw based on the first sensor signal; measure the second pitch of the second lead screw based on the second sensor signal; and simultaneously move the first and second elevator actuators based on the measured first pitch and the measured second pitch.
 13. The data storage device as recited in claim 12, wherein the control circuitry is further configured to: measure the first pitch by turning the first lead screw a predetermined number of rotations and measuring a first displacement of the actuator arm along the axial dimension based on the first sensor signal; and measure the second pitch by turning the second lead screw the predetermined number of rotations and measuring a second displacement of the at least part of the ramp along the axial dimension based on the second sensor signal.
 14. The data storage device as recited in claim 13, wherein: the second sensor signal consists of a second sinusoidal sensor signal; and after moving the first elevator actuator to the first crashstop position and moving the second elevator actuator to the second crashstop position the control circuitry is further configured to: control the second elevator actuator to move the at least part of the ramp along the axial dimension away from the second crashstop position until a zero crossing is detected in the second sinusoidal sensor signal; and measure the first displacement and the second displacement after the zero crossing is detected in the second sinusoidal sensor signal. 